The ecotoxicological impact of microplastics on freshwater invertebrates

Abstract

Microplastics (MPs) are small plastic particles released directly from the use of cosmetic products, or indirectly through the degradation of large plastic items under environmental conditions. Microplastics have been found in marine and freshwater environments around the world, raising concerns about the long term impact on animals and ecosystems in addition to recent discoveries of MPs entering the human food chain. The impacts of MP pollution on ecosystems and their functioning remain poorly quantified and there is a paucity of information on the impacts of MPs in freshwater ecosystems, despite the broad range of pathways through which MPs can proliferate and the extensive range of species which actively ingest MPs in these systems. This thesis aims to obtain key data on the uptake, fate and ecotoxicological impact of MPs on freshwater invertebrates. Initially, MP uptake and chronic toxicity tests were gathered by exposing the crustacean water flea Daphnia magna Straus 1820 (Cladocera) to polystyrene MPs of sizes 2 and 15 µm. The endpoints were mortality, growth and number of offspring. The results indicate that D. magna selectively uptake food particles over MPs, and that the toxicity was mainly linked to the availability of food. Moreover, a significant size dependent increase of toxicity was observed, with exposures to 2 µm sized particles being more toxic than 15 µm sized particles. Uptake, fate and toxicity of MPs were also studied in a holometabolous insect by exposing the common house mosquito Cu/ex pipiens Linnaeus 1758 (Diptera) to polystyrene MPs of sizes 2 and 15 µm. Results showed both particle sizes were readily taken up by larval mosquitoes then transferred to the adults via pupae. There were more transfer of MPs size 2 µm compare it to 15 µm without any effecting on mortality and weight of adults. This work is the first to demonstrate that MPs can be transferred ontogenically through organisms with complex life histories, presenting a potential pathway for dispersal of MPs into terrestrial environments. Laboratory studies were followed up with a field study exposing a small freshwater pond community to 15 µm polystyrene MPs. The results show that a high proportion of MPs accumulated in the sediment while only a small amount remained in the water column, with a significant correlation between the number of MPs in the water and the freshwater invertebrate. The presence of MPs had no real impact on the freshwater community, with season being a more important variable. Finally, the predatory ability of non-biting midge larvae, Chaoborus flavicans, towards larvae of C. pipiens mosquitoes loaded with 2 µm was quantitatively examined by linking MP trophic transfer with predation rates in a functional response framework. Results demonstrated a lack of effect of MPs on predation rates and correlation number of MPs transferred through predation.